Field of the Invention
The present invention relates generally to the use of heated fuels in internal combustion engines, such as diesel engines, and specifically to novel methods and systems for compensating for conditions detrimental to optimal engine function, for example, chemical and physical characteristics of fuels which can affect combustion and cold start ignition.
Description of the Related Art
An internal combustion engine burning diesel fuel differs from standard internal combustion engines burning other liquid fuels (i.e. gasoline) or gaseous fuels such as propane or natural gas in that it utilizes a compression-ignition arrangement to ignite the diesel fuel while a spark plug is generally used to ignite a mixture of air and other fuel. Such a compression-ignition system primarily relies on the heat of compression to initiate ignition of the fuel that has been injected into the combustion chamber. However, diesel engines tend to exhaust a significant amount of particulates, various organic compounds and NOx. Furthermore, because an excess of air is usually present, a considerable amount of carbon dioxide (“CO2”) is produced which has undesirable regulatory and environmental implications.
Additionally, various physical properties and characteristics of the diesel fuel used in the above-mentioned engines can affect the efficiency of combustion in the compression-ignition arrangement. For example, compression-ignition can be affected by fuel density, lubricity, cold-flow properties, and sulfur content. One of the most significant characteristics affecting combustion efficiency is cetane number.
Cetane number is a measure of a fuel's ignition delay (i.e. the time period between the start of injection and the first identifiable pressure increase during combustion). A more simplified way of expressing this is that cetane number is a measure of how quickly a fuel starts to burn (auto-ignites) under compression-ignition conditions. Cetane itself (also known as Hexadecane) is an un-branched alkane hydrocarbon with the chemical formula C16H34; it has combustive properties and easily ignites under the pressure of diesel compression. Pure cetane has a cetane number of 100, while the fuel constituent alpha-methyl naphthalene has a cetane number of 0. All other hydrocarbons present in a particular diesel fuel are indexed to cetane as to how well they ignite under compression. Generally, fuels with a higher cetane number will have shorter ignition delay periods than lower cetane number fuels.
Cetane number is difficult to control and varies widely between different diesel fuels, or indeed even different batches of the same diesel fuel. For example, in the United States, cetane numbers for diesel fuels typically range from 38 to 53. Due to the inconsistencies regarding cetane number, methods have been developed to attempt to compensate for the cetane number in order to improve combustion efficiency.
One such method utilized to compensate for cetane number variance is to control injection timing. However, this method is not strictly reliable and produces unwanted side effects on both the chemical and physical process. Because cetane number affects the process of combustion and the mixture formation of the resulting by-products, when injection timing is controlled in attempt to compensate for cetane number variance. This control will also alter the overall balance of various combustion concerns, including noise, exhaust gas emissions, NOx, combustion stability, fuel consumption ratios, etc.
Another conventional method of reducing the above emissions, is through engine downsizing. However, downsizing strategies require an increased combustion efficiency with a boost in pressure to increase torque and power to maintain the performance level of a non-downsized engine. This is difficult as increased pressures are limited by structural design factors which result in a limited acceptable maximum cylinder pressure. Maintaining maximum cylinder pressure while downsizing engine components and simultaneously compensating for the downsized components with increased boost pressure requires lowering the compression ratio. However, maintaining a low compensation ratio results in a higher engine weight and sacrifices cold start performance of the vehicle, particularly when the engine block becomes cooled during cold weather.
Another attempted solution is to utilize glow assist such as the use of glowplugs. Glowplugs can produce high temperature conditions and improve ignitability of the fuel. Heat generated by the glowplugs is directed into the cylinders, and serves to warm the engine block immediately surrounding the cylinders, thus warming the air in the cylinders. Typical glow assist systems have a “wait to start” pre-heating cycle that utilizes internal sensors to detect when the engine block has reached a designated temperature, the glowplug relay then switches off a “wait-to-start” light. A pre-heating cycle usually lasts for 2 to 5 seconds. The operator of the vehicle then proceeds to activate the ignition and start the engine. The glowplug relay switches off the glowplugs after the engine is running. However, such a glow system does not always reduce the production of white smoke and other combustion particles in the exhaust.
Yet another attempted solution is to utilize a block heater to increase air intake temperature. However this significantly raises the cost of producing such an engine and requires additional components that will need to be replaced should they malfunction.
An efficient method and system for compensating for variance in cetane number, or lower cetane number, as well as improving cold start performance while reducing exhaust gas emissions without generating the typical various side effects above is therefore needed.